The present invention relates to a hydrogen storage material and a process of producing the same. More particularly, it relates to a hydrogen storage material which is, while with a minimized cobalt content, excellent in insusceptibility to grain size reduction and hydrogen storage characteristics (PCT characteristics) and exhibits not only excellent initial activity that is an important characteristic for use in a battery but high output characteristics for use in electric tools, etc. or low-temperature characteristics for use in hybrid electric vehicles, and a process for producing the same.
Nickel-metal hydride storage batteries (secondary batteries) having a hydrogen storage material in the anode have recently been attracting attention as high capacity alkali storage batteries supplanting nickel-cadmium storage batteries. The hydrogen storage materials that are currently used widely are hydrogen storage alloys composed of five elements, i.e., Mm (misch metal, a mixture of rare earth elements), Ni, Al, Mn, and Co.
Compared with La-based alloys, the Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloys enable constructing an anode out of relatively cheap materials and provide closed nickel-metal hydride storage batteries having a long cycle life and a controlled inner pressure rise which is caused by gas generated in case of an overcharge and have therefore been used widely as an electrode material.
The Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloys in current use are designed to have a prolonged cycle life by preventing the alloys from reducing their grain size. It is generally known that about 10% by weight of Co (0.6 to 1.0 in an atomic ratio) is required to prevent the grain size reduction. It is also accepted that a given amount of Co is necessary for securing excellent hydrogen storage characteristics and anticorrosion.
However, the material cost increases with the Co content, which is problematical from the aspect of material cost. Taking into consideration application of the hydrogen storage material to large batteries, such as the power source of electric vehicles, and the ever expanding market of nickel-metal hydride storage batteries, in particular, the material cost is weighty in choosing anode materials and has been a matter of concern.
To settle the above problem, JP-A-9-213319 proposes altering the composition of the Mmxe2x80x94Nixe2x80x94Mnxe2x80x94Alxe2x80x94Co alloy and adding thereto a small amount of an additional element. Use of the hydrogen storage material powder disclosed therein as an anode makes it feasible to reduce the Co content and yet to suppress deterioration of the anode caused by the alloy""s reduction in grain size below a certain level and thereby to extend the cycle life of the battery.
Because the alloy of the composition disclosed in JP-A-9-323319 does not always secure stability in its characteristics, the present inventors have proposed in JP-A-11-152533 a composition and a production process for obtaining satisfactory initial activity, whereby a low-Co alloy has now come to be used in special applications.
However, where the hydrogen storage materials disclosed in the above publications (JP-A-9-213319 and JP-A-11-15253) are used, output characteristics, especially output in low temperature, are insufficient for electric tools needing high output characteristics or for hybrid electric vehicles.
Accordingly, an object of the present invention is to provide a hydrogen storage material of which the production cost is reduced by extremely decreasing its cobalt content and which exhibits excellent insusceptibility to grain size reduction, excellent hydrogen storage characteristics, satisfactory output characteristics, and satisfactory storage characteristics and a process for producing the same.
As a result of extensive studies, the present inventors have found that the above object is accomplished by a hydrogen storage material of AB structure having a specific nonstoichiometric composition (B site rich), particularly a composition having 4.1 less than Ni£4.3 and 0.4 less than Mn£0.6, and the c-axis of which is in a given range. They have also found that such a hydrogen storage material is obtainable with the above-described specific composition when a casting temperature and heat treating conditions satisfy a given relationship.
The present invention has been reached based on the above findings and provides a hydrogen storage material which is an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure represented by general formula:
MmNiaMnbAlcCod
wherein Mm is a misch metal, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, and 5.2xe2x89xa6a+b+c+dxe2x89xa65.45,
or general formula:
MmNiaMnbAlcCodXe
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, 0 less than exe2x89xa60.1, and 5.2xe2x89xa6a+b+c+d+exe2x89xa65.45,
characterized in that the lattice length on the c-axis is 406.2 pm or more.
The present invention also provides a preferred process for producing the hydrogen storage material of the present invention which comprises heat-melting hydrogen storage alloy raw materials, casting the melt, and heat treating the resulting alloy in an inert gas atmosphere to produce an AB5 type hydrogen storage material having a CaCu5 type crystal structure represented by the following general formulae, characterized in that the casting temperature is 1350 to 1550xc2x0 C., the pouring temperature is 1200 to 1450xc2x0 C., and conditions of said heat treating are 1040 to 1080xc2x0 C. and 1 to 6 hours.
General formula:
MmNiaMnbAlcCod
wherein Mm is a misch metal, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, and 5.2xe2x89xa6a+b+c+dxe2x89xa65.45,
or general formula:
MmNiaMnbAlcCodXe
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, 0 less than exe2x89xa60.1, and 5.2xe2x89xa6a+b+c+d+exe2x89xa65.45.
The Best Mode For Carrying Out The Invention:
The hydrogen storage material according to the present invention is an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure represented by general formula:
MmNiaMnbAlcCod
wherein Mm is a misch metal, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, and 5.2xe2x89xa6a+b+c+dxe2x89xa65.45,
or general formula:
MmNiaMnbAlcCodXe
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1 less than axe2x89xa64.3, 0.4 less than bxe2x89xa60.6, 0.2xe2x89xa6cxe2x89xa60.4, 0.1xe2x89xa6dxe2x89xa60.4, 0 less than exe2x89xa60.1, and 5.2xe2x89xa6a+b+c+d+exe2x89xa65.45.
In the above formulae, Mm is a misch metal, a mixture of rare earth elements such as La, Ce, Pr, Nd, and Sm. The hydrogen storage material is an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure having a B site-rich nonstoichiometric composition ranging from AB5.2 to AB5.45.
In this hydrogen storage material, the compositional ratio (atomic ratio) of NiaMnbAlcCod fulfills the following relationships. The ratio of Ni: 4.1 less than axe2x89xa64.3. The ratio of Mn: 0.4 less than bxe2x89xa60.6. The ratio of Al: 0.2xe2x89xa6cxe2x89xa60.4. The ratio of Co: 0.1xe2x89xa6dxe2x89xa60.4. (a+b+c+d) is in a range of from 5.2 to 5.45.
The compositional ratio (atomic ratio) of NiaMnbAlcCodX, (wherein X is Cu and/or Fe) satisfies the following relationships. The ratio of Ni: 4.1 less than axe2x89xa64.3. The ratio of Mn: 0.4 less than bxe2x89xa60.6. The ratio of Al: 0.2xe2x89xa6cxe2x89xa60.4.The ratio of Co: 0.1xe2x89xa6dxe2x89xa60.4. The ratio of X: 0 less than exe2x89xa60.1. (a+b+c+d+e) is in a range of from 5.2 to 5.45.
As described above, the ratio of Ni, a, is more than 4.1 and up to 4.3, desirably from 4.15 to 4.25. If a is 4.1 or less, the output characteristics are not satisfactory. If it exceeds 4.3, deterioration in insusceptibility to grain size reduction or life characteristics is observed.
The ratio of Mn, b, is more than 0.4 and up to 0.6. If b is 0.4 or less, the plateau pressure increases, and the hydrogen storage capacity is reduced. If it exceeds 0.6, the alloy undergoes considerable corrosion so that the battery voltage greatly decreases during storage.
The ratio of Al, c, is from 0.2 to 0.4. If c is smaller than 0.2, the plateau pressure, which is the hydrogen release pressure of a hydrogen storage material, increases to deteriorate energy efficiency in charges and discharges. If it exceeds 0.4, the hydrogen storage capacity is reduced.
The ratio of Co, d, is 0.1 to 0.4. If d is less than 0.1, the hydrogen storage characteristics or the resistance to grain size reduction are deteriorated. If it exceeds 0.4, the proportion of Co increases, failing to realize cost reduction.
The ratio of X, e, is from 0 to 0.1. If e is more than 0.1, the output characteristics are impaired, and the hydrogen storage capacity is reduced.
(a+b+c+d) or (a+b+c+d+e) (these sums will hereinafter be sometimes referred to as x, inclusively) is from 5.2 to 5.45. If x is smaller than 5.2, the battery life and the resistance to grain size reduction are ruined. If x is greater than 5.45, the hydrogen storage characteristics are reduced and, at the same time, the output characteristics are also deteriorated.
The hydrogen storage material of the present invention has a lattice length on the c-axis of 406.2 pm or more, preferably 406.6 to 407.1 pm. If the lattice length on the c-axis is shorter than 406.2 pm, the alloy has poor insusceptibility to grain size reduction and reduced battery life characteristics.
The c-axis lattice length of the hydrogen storage material has different preferred ranges according to the value of (a+b+c+d) or (a+b+c+d+e), i.e., the value x. The value x being 5.02 or greater and smaller than 5.3, the c-axis lattice length is preferably 406.2 to 406.8 pm. The value x ranging from 5.3 to 5.45, the c-axis lattice length is preferably 406.8 to 407.3 pm.
Although the lattice length on the a-axis of the hydrogen storage material of the present invention is not particularly limited, it is usually from 500.5 to 501.2 pm.
The process of producing the hydrogen storage material of the present invention is then described. Raw materials of the hydrogen storage material are weighed to give the alloying composition described above and mixed up. The mixture is melted into a melt by means of a high frequency induction furnace based on induction heating. The melt is poured into a casting mold, for example, a mold of water cooling type at a casting temperature of 1350 to 1550xc2x0 C. to obtain a hydrogen storage material. The pouring temperature is 1200 to 1450xc2x0 C. The term xe2x80x9ccasting temperaturexe2x80x9d as used herein means the temperature of the melt in the crucible at the beginning of casting, and the term xe2x80x9cpouring temperaturexe2x80x9d means the temperature of the melt at the inlet of the casting mold (i.e., the temperature of the melt before entering the casting mold).
The resulting hydrogen storage material is heat treated in an inert gas atmosphere, for example, in argon gas under heat treating conditions of 1040 to 1080xc2x0 C. and 1 to 6 hours. A cast alloy structure usually shows fine grain boundary segregation chiefly of Mn. The heat treatment is to level the segregation by heating.
There is thus obtained a hydrogen storage material which has a reduced cobalt content and yet exhibits excellent insusceptibility to grain size reduction, excellent hydrogen storage characteristics, satisfactory output characteristics, and satisfactory storage characteristics.
The hydrogen storage material is crushed, pulverized, and subjected to surface treatment, such as an acid treatment, an alkali treatment or a like treatment and is suitably used as an anode of high-output alkali storage batteries. The alkali storage batteries thus provided are satisfactory in initial characteristics and low-temperature high-output characteristics. The anode made of the hydrogen storage material is prevented from deterioration due to the alloy getting finer and therefore secures a long cycle life.